Preheat an outdoor grill for medium-high heat. When grill is hot, lightly oil the grate.
In a large bowl, mix together the ground beef, tuna, onion, and relish. Season with seasoning salt and pepper to your liking. Form the meat mixture into 6 patties.
Place patties on the hot grill, and cook for 6 to 9 minutes on each side, or until cooked through. Serve on buns with the usual toppings.